The present invention relates to a pattern forming process. More particularly, the invention relates to a pattern forming process capable of giving a fine pattern with a high accuracy even when the surface of a substrate has topography (unevenness).
As the scale of a semiconductor integrated circuit and its density are being increased recently, formation of an extremely fine pattern with a high accuracy is demanded more and more eagerly.
As is well known, a process called "photolithography" is generally employed in forming fine patterns in semiconductor integrated circuits, magnetic bubble memory devices, etc. An excellent resist pattern having a line width of about 1 .mu.m can be formed according to the photolithography when a substrate having a smooth surface and a low reflectivity is used. However, when the surface of a substrate is uneven and reflective pattern deformation occurs because of reflection of the incident light from the topographic surface of the substrate.
Further, in the latter case, a difference in the film thickness of the resist layer on the substrate occurs between the upper level part and the lower level part and, therefore, the light interference varies in the resist film to increase or decrease the pattern width, to thereby reduce the dimensional stability of the pattern. To effect the patterning in an excellent manner by solving these problems due to such unevenness of the substrate surface, there has been proposed a multi-layer resist process.
The multi-layer resist process comprises applying a thick layer of an organic substance having a high absorbance onto the whole surface of a substrate to flatten the surface, forming a thin resist pattern thereon according to a known photolithographic technique, transferring the resist pattern to said thick layer of the organic substance and etching exposed parts of the substrate. In one embodiment, the exposed parts of the above-mentioned thick organic layer are etched directly using said resist pattern as a mask. In another embodiment, a thin film (intermediate layer) made of a material having a high dry etching resistance is interposed between the thick organic substance layer and the resist pattern, unnecessary parts of the thin film are removed and then the exposed parts of said thick organic substance layer are etched using the thin film as a mask. Since the topography or unevenness of the substrate surface is eliminated by forming the layer of the organic substance having a high absorbance, a light penetrating through the photoresist film formed thereon is absorbed by the layer of the organic substance. Therefore, the phenomena of halation on the uneven part of the substrate and the light interference in the resist film in the uneven portion of the substrate due to the reflection and scattering of the light can be reduced. As a result, the deformation of the pattern can be minmized and the dimensional stability can be improved. The organic substances used usually heretofore in eliminating the topography include positive novolak resin resists and polyimide resins. To improve the absorbance of the organic substances, a process comprising a thermal treatment thereof at a temperature of 200.degree. C. or higher or addition of a dye having a high absorbance thereto has been proposed (see the specification of Japanese Patent Laid-Open No. 172736/1982).
However, in order to absorb a light of 436 or 405 nm, which is an exposure wavelength currently employed in a reduction projection exposure process using said novolak resin positive resist as a material of bottom organic substance layer, a baking at a temperature of as high as 200.degree. C. or above is necessary after the formation of the bottom layer. The higher the baking temperature, the higher the absorption of the light of exposure. However, as the baking temperature is elevated beyond 200.degree. C., the surface of the bottom organic substance layer begins to be carbonized to damage the adhesion between the organic substance layer and the resist film formed thereon.
It was also reported that a novolak resin positive resin containing a light-absorptive dye was tentatively used as a material for the bottom layer. However, the baking temperature cannot be elevated beyond about 160.degree. C., since the light-absorptive dye sublimes or decomposes at such a high temperature, as described by Michel M. O' Toole et al. in "Linewidth Control in Projection Lithography Using a Multilayer Resist Process" IEEE Electron Device, Vol. ED-28, p. 1405 (1981). Consequently, the bottom layer film is insufficiently insolubilized so that it is dissolved when the top photoresist layer is applied thereto, thus making the formation of the multi-layer structure difficult. Further, in forming an intermediate layer between the top layer and the bottom layer by applying spin-on glass (SOG) thereto, a gas is generated from the bottom layer when the intermediate SOG layer is baked to form SiO.sub.2, because the baking temperature of the bottom layer was low. As a result, cracks are formed in the SOG layer to make the formation of the multilayer film difficult.